For example, in a substrate processing apparatus for performing a process, such as plasma etching and the like, on a semiconductor wafer by using a plasma generated in a substrate processing chamber, temperatures of the semiconductor wafer and various components provided in the substrate processing chamber are measured and process conditions such as plasma generation conditions and the like are controlled to secure processing precision.
As one example, there has been proposed a temperature measuring method for measuring a temperature of a focus ring, which greatly affects a plasma distribution on a semiconductor wafer, by performing Fourier transform on a distribution of intensity of lights reflected at front and rear surfaces of the focus ring when the focus ring is irradiated with a low-coherence light in the thickness direction thereof (see, e.g., Japanese Patent Application Publication No. 2013-029487 (JP2013-029487A)).
As another example, there has been proposed a temperature measuring method for measuring a temperature of a focus ring including a thin portion at a rear surface side thereof and a coating member which is disposed with a predetermined space between the thin portion and the coating member, by using an interference light between each of reflection lights at front and rear surfaces of the thin portion when the thin portion is irradiated with a low-coherence light in the thickness direction thereof and a reflection light obtained by irradiating a low-coherence light on a reference mirror (see, e.g., Japanese Patent Application Publication No. 2012-204742 (JP2012-204742A)).
Both techniques described in JP2013-029487A and JP2012-204742A use thickness change of a focus ring according to the temperature thereof by an effect of coefficient of thermal expansion. That is, a relationship between the thickness of the focus ring and the temperature thereof is previously measured, the thickness of the focus ring is obtained by using a low-coherence light, and the obtained thickness of the focus ring is compared with the previously measured relationship between the thickness of the focus ring and the temperature thereof, whereby the temperature of the focus ring can be measured.
However, in the technique of JP2013-029487A, when the front or the rear surface of the object to be temperature-measured, i.e., the focus ring, to which a low-coherence light is irradiated is worn or is deposited with a foreign material, it is difficult to measure a thickness of the object and, thus, a precise temperature of the object may not be obtained. In contrast, in the technique of JP2012-204742A, since states of the front and the rear surfaces of the thin portion are not changed, precise temperature measurement is possible. However, there is a drawback in which a cost may increase due to the increase in the number of components and a precise temperature measurement may not be ensured when an assembly accuracy gets worse.